Traditional solid state imaging devices, such as those based on CCD (charge-coupled device) and CMOS (complementary metal oxide semiconductor) imaging devices, typically consist of a two-dimensional array of photosensors that are distributed on a surface or layer of a semiconductor chip, as well as an optical system used to focus an image (in the form of light passing through an aperture) onto the array. Each photosensor of the array commonly is generally referred to as a “picture element” or a “pixel.” The amount of light energy reaching each photosensor is recorded and stored, and the output of the photosensors, in the aggregate, forms a captured image. Such imaging devices, or “imagers,” can be configured to capture either gray scale images or color images. Color imagers are generally configured to have groupings of adjacent red, blue and green pixels forming the photosensor array.
It is generally desirable for the optical system of such imagers to collect as much light as possible while still providing the largest possible depth of field in the produced image. The phrase “depth of field,” as used herein, refers to the areas of an image that remain in focus both in front (i.e., closer to the photosensor array) of, and behind the main focus point of the optical system. Depth of field can be affected by the aperture of the optical system and by the distance to the object being imaged, with a closer object producing a shallower depth of field, as well as with shorter focal lengths producing a greater depth of field.
An optical system's numerical aperture (“NA”) is the controlling feature that governs the total amount of light available to the imager, and is generally defined as a ratio of an aperture of a lens of the optical system to the focal length of the lens. Mathematically speaking, the numerical aperture, NA, can be expressed as follows:NA=½(d/f)wherein d represents the diameter of the aperture opening and f is the focal length of the lens. In a digital imager, the focal length, f refers to the optical distance between the lens assembly and the surface of the photosensor array when a desired image is focused onto the array.
The depth of field of an imager and the brightness of an image captured by an imager are functions of NA and of the number of photosensors which provide the image's spatial resolution. These parameters are interrelated to effectively require a trade-off between the brightness of a captured image and the depth of field of the image. Put another way, in existing imagers, bright images are desirable (to illuminate visual details within the image); however, the brighter the image, the smaller the depth of field, and vice versa.
This trade-off is readily illustrated in, for example, a conventional CCD-type endoscope or borescope, where illumination is usually limited by environmental conditions, thus favoring a design with an aperture that is large enough to provide a usable amount of light and to impart brightness, but also small enough so that sufficient depth of field is provided for a specific application. Often times, such a compromise sacrifices the best of both worlds, resulting in a dim image with poor depth of field.
Another drawback associated with conventional imagers arises from the complex and delicate (usually mechanical) systems required to move the lens of the optical system and to change the light-admitting aperture of the optical system.
Thus, a need exists for an electronic imaging device that allows for the capture and display of images that are both bright and of a high depth of field.